Ice making machines are employed in commercial and residential applications around the world. In domestic applications, ice makers are typically located in a freezer compartment. The resulting ice is usually of poor quality due to the trapping of air and impurities during the freezing process. In commercial applications, the ice makers typically freeze the ice upright, or vertically, in a manner that removes the impurities and creates pure, clear ice cubes. Among other references, U.S. Pat. No. 5,237,837 and Patent Publication No. 2010/0251746 are known and explain the embodiments of this process in detail. Commercial ice makers traditionally consist of a single ice making unit placed above an ice storage bin or automatic dispenser for accessing the ice. An ice level sensor signals when the bin or dispenser level is full, at which point, the ice making unit shuts down until the demand returns. As ice is dispensed or drawn from the bin, the ice falls away from the sensor and production resumes. U.S. Patent Publication No. 2008/0110186 is known and further explains this process in detail. Such machines have received wide acceptance and are particularly desirable for commercial installations such as restaurants, bars, motels and various beverage retailers having a high and continuous demand for fresh ice.
The refrigerant selection is a key element in the design of the ice maker. Ice machine evaporators operate at a medium to low temperature, having an optimum temperature ranging from −10° C. to −20° C. In September 1987, the Montreal Protocol banned the use of CFCs and began the phase-out of R-22. In its place, non-ozone depleting HFC refrigerants became the standard for the ice making application. In particular, R-404a, the pseudo-azeotropic blend of HFC-125, HFC-143a, and HFC-134a, provides a nearly stable temperature throughout the evaporation process, which is critical to producing a consistent ice slab across an evaporator. It is also non-flammable and, therefore, has no charge limitation placed on its use in commercial ice making machines. Higher ice capacities are possible by simply increasing the size of evaporator, compressor, and condensing unit, and in turn, increasing the amount refrigerant necessary to provide the proper charge for the system. Larger ice makers with self-contained condensing units could contain as much as 5 pounds (2,268 grams) of R-404a, and systems with remote condensing units could have over 10 pounds (4,536 grams) of R-404a, depending on the length of the connecting line sets.
Despite its optimum fit for the application, R-404a is receiving increasingly negative attention about its effect on the environment. GWP is the measure of given mass of greenhouse gas that is estimated to contribute global warming. Its relative scale is compared to that of Carbon Dioxide (CO2) gas, which by convention has a GWP of one. R-404A is estimated to have a GWP of 3,922. Its direct release to the atmosphere is prohibited, however, the indirect release of refrigerant over the life of the equipment due to infinitesimal leakage can be nearly impossible to ascertain. An even greater impact exists with the indirect effect of the increased energy consumption required of equipment running on a reduced charge. In this case, —the impact is manifested with increase in carbon emissions released to the atmosphere during the creation of that additional energy. As such, the phase-out of HFC refrigerants has gained worldwide momentum. The European Union has taken measures to cut two-thirds of the emissions from fluorinated greenhouse gasses by 2030 by passing “F-gas Regulations,” which took effect January 2015. The United States has followed suit by passing similar phase-out schedules to take effect as early as January 2016. Individual states have taken up the challenge as well. Specifically, the state of California proposed a rule in June, 2015, to ban all refrigerants with a GWP greater than 150 by January, 2021. To date, there are several alternative refrigerants which offer a potential drop-in replacement, such as R-407A or HFO blends like R-448, but none are below California's 150 GWP limit. Also, in particular for ice making machines, it is a requirement that any alternative working fluid have a negligible temperature glide in order to make ice evenly over the evaporating surface. The aforementioned HFO blends have a comparatively high temperature glide which make them unsuitable for the application. Ice maker manufactures will have no choice but to comply with the new laws taking shape, and ultimately, there will be an end to the use of HFCs and the proposed HFO alternative blends, and the ice making equipment will need to be completely redesigned.
With the aforementioned phase-out facing ice making manufacturers, the case for natural refrigerants has never been so prevalent. Propane (R-290) is a highly efficient and very environmentally friendly alternative having a GWP of only 2. It can essentially be dropped into existing systems without major modification; however, R-290 poses its own set of design challenges due to its flammability. The IEC has imposed a refrigerant charge limit of 150 grams in an effort to mitigate that risk. To take advantage of the benefits of R-290, manufacturers must develop techniques to limit the refrigerant charge of the system. One such technique is explained in U.S. Pat. No. 9,052,130, where a traditional fin and tube condenser has been replaced with an equivalent microchannel condenser with an internal volume from 100 to 250 milliliters. However, microchannel condensers are traditionally more expensive than fin and tube condensers, and with a volume of only 250 ml, there still remains a limit on the maximum ice capacity that can be obtained with such a condenser. Ice manufacturers have successfully made 500 pounds of ice per day with 150 grams of propane, but no solution exists for icemakers requiring greater capacity in a single system. Logically, to achieve the higher ice capacities, those skilled in the art would then be lead to employ multiple systems into one machine. U.S. Pat. No. 4,384,462 discloses a multiple compressor system that includes a plurality of evaporators and expansion devices that responds advantageously to increasing demand by cycling the systems according to that demand. Although not directly related to ice making machines, one could imagine a similar system for a commercial ice maker that would respond similarly to ice demand. However, the cost of multiple systems would make the product unprofitable. The evaporator, being made of a high thermal conductive material such as copper, is in some cases the most expensive component of an ice making machine. Outside of material cost, the fabrication, overhead costs, and any additional cost of performance coatings, such as Electroless Nickel, can sum to as much as a third of the entire ice making machine material cost. There could also be some significant performance-related drawbacks. A dual-evaporator system with cycling control would scale or corrode one evaporator more rapidly than the other resulting in more frequent failures of one side, effectively reducing the ice making capacity in half. The increased warranty costs for a hydrocarbon dual evaporator system drastically effect the business case and consume any potential profits as compared with the single HFC system evaporator standard of the day. Therefore, the current solutions presented for R-290 unfortunately offer little solution for larger ice making machines in a competitive market driven to reduce overall costs, especially with emerging manufacturers from around the world offering new competition.
A single R-290 system ice maker still offers the best solution, as it reduces the number of required components and conserves cost, but there must be a means to increase the ice capacity without significantly adding refrigerant charge. Although not specifically intended, one method that could be incorporated is the one described in U.S. Pat. No. 7,017,355, which uses two evaporator freeze plates with one refrigeration circuit. A rectangular cross-sectioned conduit is used between the two evaporator plates, increasing the efficiency of the system by recovering the heat traditionally lost on the opposite side of the refrigerant tubing. However, this method is unproven in the market and there is little evidence that flat conduit would last the duration of the icemakers service life due to the high probability of plate-tube separation. Surface imperfections in the flatness would cause pockets of air between the plate and tube, and ultimately lead to the build-up of ice between the two surfaces. Over repeated thermal cycling, the ice would expand to propagate behind the freeze plate, which lead to a reduced ice capacity and ultimately complete failure. On the contrary, ice making evaporators with round tubing attached to the freeze plate surface has been proven superior to the flat conduit by withstanding 10 or more years of thermal cycling without separation.
Thus, need remains for a single, commercial ice making machine capable of making more than 500 pounds of ice per day and that uses R-290 as its refrigerant. The solution demands that (1) the individual systems adhere to limitations set in place for hydrocarbons, (2) manufacturing costs be limited by reducing the number of expensive components and systems, and (3) a proven and reliable method to produce an evaporator can be repeated with good adhesion to the freeze plate. The present disclosure allows higher ice capacities in the event the charge limitations increase for R-290 single systems beyond 150 grams. Nonetheless, there will always be a charge limitation for use of flammable refrigerants for commercial equipment located and installed indoors. Those skilled in the art will have determined the maximum allowable ice capacity given the refrigerant limit, and in this case, the essence of the present disclosure in allowing still higher ice capacities would still apply.